Eating M&M's was a big deal for Nicholas Vanech when he was 10 years old. He was allergic to tree nuts (almonds, etc.), so candy was always a gamble for him, but the chocolate candy label only said "may contain peanuts," which he knew he wasn't allergic to. So he popped them in his mouth.

It wasn't long before Nicholas, now 18, went into anaphylactic shock, a severe allergic reaction that causes the tongue and throat to swell, making it difficult to breathe and requiring a trip to the emergency room.

"I called [the candy manufacturer] after my son's reaction," said Denise Vanech, Nicholas's mother."I said, 'Is it possible that he could have been exposed to tree nuts from your product?' They said, 'Yeah. It's the same facility.'"

That was nearly a decade ago.

Since then, food labeling in the United States (and at Mars, Inc.) has undergone significant changes to prevent customers from experiencing allergic reactions, but there's still a long way to go, experts say.

"It's a very difficult topic to find a perfect solution for," said Dr. Scott Sicherer, a professor and researcher at the Jaffe Food Allergy Institute at Mount Sinai Hospital in New York. "Companies can always make a mistake and have recalls if a product is found to have a mislabeled an ingredient in it. Unless someone gets sick from it, they wouldn't know it was there."

The Food and Drug Administration and manufacturers have issued 20 recalls in the last 60 days for undeclared allergens in food products, including Chicken of the Sea tuna, which had undeclared soy; two kinds of Wegmans brownie mix with undeclared milk; and two kinds of ice cream with undeclared pecans, according to FDA records.

An ABC News analysis found more than 400 recalls for undeclared allergens in food reported to the FDA since March 2009. More than 140 of them were for desserts and snack foods, like cookies, candy and ice cream. Repeat brand recalls were often from grocery stores, such as Kroger, Publix, Whole Foods Market and Wegmans.

Eggs, fish, milk, peanuts, shellfish, soy, tree nuts, and wheat make up 90 percent of food allergies, according to a 2008 CDC report that found an 18 percent rise in children diagnosed with food allergies between 1997 and 2007.

Federal law requires manufacturers to list the top eight allergens on food labels in plain English -- "milk" instead of "casein," for example. The law, called the Food Allergen Labeling and Consumer Protection Act of 2004, went into effect in 2006.

But other allergens, like sesame, don't have to be called out on food labels. It's also possible that products that don't contain an allergen can become contaminated with it if the allergen-free product is made or packaged in the same factory as a product containing that allergen.

"Advisory labeling, some people call this precautionary labeling," Sicherer said, "those types of comments are totally voluntary. They're not part of the law."

As such, the labels aren't consistent with one another. Labels can say "may contain peanuts," or "made in a facility that also processes nuts," which many allergic people find confusing. It's possible a manufacturer failed to write an advisory or that the manufacturer over-labeled on items that were at low risk for contamination.

Food safety lawyer Bill Marler said he's never had a case in which someone had an allergic reaction because a manufacturer failed to say it intentionally put nuts or milk in its product, but he's had cross-contamination cases.

"I think there's probably a lot more of these not-straight-up labeling issues," he said. "I do think the manufacturing process is open to mistakes being made, especially when they're making multiple types of food in the same facility."

In 2001, Marler fought for a Seattle boy whose mother had to hold him down as emergency doctors gave him two shots of epinephrine. The boy had eaten a health bar that was supposedly milk-free, but he soon became nauseous and broke out in hives. His mother re-read the label looking for milk, to which her son was severely allergic, but it wasn't there.

Marler said he also remembered an old case in which a fruit basket was nut-free but caused an allergic reaction because nut dust from another fruit basket contaminated it.

"It didn't kill the kid, but it came really close," Marler said.

Dr. Donna Hummell, an allergist at the Monroe Carell Jr. Children's Hospital at Vanderbilt University said chocolate and candy are often especially risky foods for people with nut allergies to eat.

"If you can buy it with almonds in it or buy it plain or buy it with peanuts in it, it's better to watch out," Hummell said.

You don't need to tell that to Nicholas's mom. Vanech has made countless phone calls to food manufacturers over the years to keep her son safe.

She can tell you that her son can eat pretzel M&M's because they're not made in the same facility as almond M&M's. She knows that Ben and Jerry's always makes chocolate, vanilla and strawberry ice cream before making flavors with nuts, but since Nicholas is allergic to eggs, too, he can't eat their vanilla. She also knows that Breyers ice cream always sanitizes machinery between flavors, so their egg-free vanilla is safe for Nicholas to eat.

"It's not a joke," Nicholas wrote in an email."I get frustrated when someone passes off my allergy as some type of intolerance or tries to coax me into eating something."

When it comes to food labels, Nicholas's advice to younger children with new food allergies is to "READ THEM," he typed, adding that it's not worth the risk to go into anaphylactic shock. He said he never leaves home without an EpiPen and reads ingredients even before he buys food.

"I think the best advice I can give to younger children with food allergies is to not to be afraid to speak up about your allergies," he wrote. "Don't be embarrassed; you haven't done anything wrong and, yes, it is OK to tell a friend or a waiter/waitress more than once if they don't seem to 'get it.'"

Mars, Inc., which manufactures M&M's, was not immediately available to comment on its manufacturing processes at the time of Nicholas's allergic reaction.